Light-emitting transistors combine the visible light emission properties of light emitting diodes (LEDs) with the switching properties of transistors. As driving elements, lateral- (e.g., field effect transistor (FET)) and vertical- (e.g., static induction transistor) type transistors have been proposed. Lateral-type transistors using a standard FET structure require a high drive voltage due to factors such as a relatively long channel length, low luminance efficiency, and small aperture ratio. Vertical-type transistors using an organic static induction transistor have relatively high currents and high speeds with low operational voltages, but the fabrication of a fine gate structure has been conventionally required to achieve a high on/off ratio.
In conventional nanotube network based field effect transistors, the nanotube network is directly contacted on two sides by metallic source and drain electrodes. To observe a significant gate induced modulation of the current through the nanotubes, between the source-drain electrodes, the surface density of the conventional nanotube network needs to lie very close to its percolation limit. This is because the nanotubes of the networks of field effect transistors are a mixture of semiconducting and metallic nanotubes, while only the carrier density of the semiconducting nanotubes are appreciably modulated by the gate field. If the nanotube surface density lies well above the percolation threshold, there are numerous purely metallic nanotube current pathways between the source and drain electrodes. This results in substantial source-drain current even when the gate field modulates the semiconducting nanotubes to minimize their conductance (the “off” state). When the gate field maximizes the semiconducting nanotube conductance (the “on” state), the overall source-drain conductance does increase. However, if the nanotube surface density is well above the percolation threshold, the increase in the “on” state current is only a fraction of the “off” state current. It is only when the nanotube surface density is very near the percolation threshold and the great majority of what would otherwise be purely metallic nanotube current pathways are interrupted by semiconducting nanotubes that the “on” state current can be orders of magnitude greater than the “off” state current.
Thin film transistors (TFTs) provide the drive circuitry for present and emerging active matrix displays including liquid-crystal and organic-light-emitting display technologies. The dominant active semiconductor in these devices is amorphous silicon, however the promise of inexpensive, solution based processing techniques, inkjet patterning and construction on flexible plastic substrates has focused much research over the past 20 years on organic semiconductors as replacements. There now exist a broad range of small molecule organic and polymeric compounds that have demonstrated transconductance. Unfortunately, the electronic mobilities of these compounds, which were initially about 5-6 orders of magnitude too low to be commercially useful, remain about an order of magnitude too low. Such low mobility can be compensated for by bringing the source and drain electrodes closer together, reducing the semiconductor channel length (CL in FIG. 7A), but that greatly raises the cost of patterning the devices, removing much of the motivation.
A new TFT architecture was disclosed in Ma et al., Appl. Phys. Lett. 2004, 85, 5084, to circumvent mobility limitations of present organic semiconductors. The device relies on an ultra thin (<20 nm) aluminum source electrode that required careful partial oxidation. While the optimized device exhibited ˜6 orders of magnitude current modulation, the low work function aluminum source electrode required an n-type active channel, restricting that device to the use of C60 as the channel material. Li et al., Appl. Phys. Lett. 2007, 91, 083507 disclosed the use of the organic semiconductor pentacene; but requires an additional 7 nm vanadium oxide layer atop the partially oxidized aluminum source electrode. As forming a partly oxidized, ultra thin aluminum source electrode is difficult to produce commercially, constrains the choice of the organic active layers, and is susceptible to electromigration; hence, limiting the device lifetime other modes of forming an electrode that does not shield the gate field is desired. Thus, there remains a need for a light emitting transistor that is easy and efficient to manufacture and can use a simplified electronics drive scheme at low operational voltages, thereby requiring less energy consumption and providing for a longer and more reliable device lifetime.